WASHINGTON — Even before President Obama strode into the Rose Garden on Thursday to announce a historic understanding to restrain Iran’s nuclear program, an elaborate White House salesmanship effort for the deal was underway.

Mr. Obama briefed congressional leaders of both parties by phone, promising more detailed explanations and plenty of consultations in the weeks to come. After the announcement, he spoke for nearly an hour from Air Force One with Prime Minister Benjamin Netanyahu of Israel, who later said that the deal would threaten his country’s survival.

Senior members of Mr. Obama’s team have fanned out to promote the accord. Vice President Joseph R. Biden Jr. phoned former Senate colleagues. Denis R. McDonough, the White House chief of staff, and Susan E. Rice, the national security adviser, also reached out to members of Congress. Colin Kahl, Mr. Biden’s national security adviser, called American Jewish groups, emphasizing that no sanctions would be lifted without proof that Iran was complying. And Antony J. Blinken, the deputy secretary of state, made the rounds of the Friday morning news programs.

The intensity of the campaign reflects the steep challenge Mr. Obama faces in building support among lawmakers for an agreement with the potential to become one of the most important American foreign policy achievements in decades. Congress, where skepticism about an Iran deal runs high and is increasingly bipartisan, views the agreement as another example of Mr. Obama’s unwillingness to engage with lawmakers on consequential policy issues.

In the weeks ahead, the White House must figure out a way to head off a move supported by Republicans and some Democrats to vote on the deal, a step that the president fears could be its death knell.

“The president has a big job here, and it’s going to be tough,” said Senator Jeff Flake, Republican of Arizona, a member of the Foreign Relations Committee. “Obviously, Congress knows that it has an important role to play, and the administration is reluctant to concede that.”

The White House says that now that it has the specific outlines of a deal, it will have an easier time allaying lawmakers’ concerns. It plans to shower them with technical detail and essentially argue that the accord is the only alternative to war.

“People were understandably skeptical because we did not have a deal to point to, but now that we do, it has strengthened our hand in dealing with Congress,” said Josh Earnest, the White House press secretary. “As they take a look at the agreement and the level of detail there is, it strengthens our case that not only are we achieving the aims we set out to achieve, but that we have a way of verifying it.”

But beyond the substance, the White House plans to make a more challenging argument: that it is simply not Congress’s place to vote on the agreement.

“We’ll make the case that this is a good deal, that this is clearly within the purview of the presidency, and this is clearly in the best interest of the country,” Mr. Earnest said.

White House officials assert that while many lawmakers have legitimate questions about the deal, some merely want to kill it. And they say that allowing lawmakers to have approval power would put American negotiators in an untenable position with Iran and international partners. “You get into sort of a back-seat driving situation,” Mr. Earnest said.

But the White House may not be able to avoid it. A handful of Democrats have signed on to a measure calling for review of the deal, sponsored by Senator Bob Corker, the Tennessee Republican who is the chairman of the Foreign Relations Committee, pushing the bill close to a veto-proof majority.

Efforts by Mr. Obama and top officials to head off such a legislative mutiny will face their first major test with an April 14 meeting of the Foreign Relations panel to revise and vote on the bill.

They see an opening to persuade Democrats such as Senator Tim Kaine of Virginia who support the review measure as a way of protecting congressional prerogatives but also back an Iran nuclear accord. In a statement on Thursday, Mr. Kaine called the deal a “positive step,” but said he would continue efforts to build bipartisan support for the review bill.

“It’s going to be the world’s best-funded, most relentless sales job that we’ve ever seen,” said Mark Dubowitz, the executive director of the Foundation for Defense of Democracies, a fierce opponent of an Iran deal. “If there’s one thing this White House can do, they know how to run a political campaign, and they are going to need it here.”

One potential compromise being discussed is to have a nonbinding vote by Congress that would register disappointment or even opposition over the deal but would not block it, said Jon B. Alterman, the director of the Middle East program at the Center for Strategic and International Studies.

“What happens if the world agrees and Congress doesn’t?” Mr. Alterman said. “Suddenly, the United States becomes quite isolated and Iran ceases to be isolated. The argument is, ‘Do you really want to be the ones to let Iran off the hook?’ ”

But that message, which Mr. Obama delivered in the Rose Garden on Thursday, could backfire, critics warned.

“They’ve continued to repeat the president’s binary choice of, ‘It’s between this deal and war,’ but they need to be cautious because that has already alienated and infuriated many of their natural allies,” Mr. Dubowitz said. “You’ve got to engage your skeptics rather than maligning them as warmongers.”

One aspect of the sales effort is a bid to assuage lawmakers’ concerns about Israel’s safety.

American Jewish leaders noted that Mr. Obama had pledged on Thursday that there would be “no daylight” between the United States and Israel on security, a phrase they saw as deliberately chosen. At a meeting at the White House in 2009, Mr. Obama said that eight years of “no daylight” between the United States and Israel had yielded no progress in the Israeli-Palestinian conflict, suggesting that a different approach was needed.

“It was significant that the president used that phrase,” said Nathan Diament, the executive director of the Orthodox Union Advocacy Center. “We see that as telegraphing a willingness to work very closely with Israel to construct security arrangements that will be important for them. That will be a critical component of the political and messaging sales job that the administration has ahead of it.”

Peter Baker contributed reporting.

A version of this article appears in print on , on Page A8 of the New York edition with the headline: With Iran Deal in Hand, White House Makes Sales Pitch to Preserve It. Order Reprints | Today’s Paper | Subscribe